Three axis inertial measurement unit with counterbalanced mechanical oscillator

ABSTRACT

An inertial measurement unit capable of providing information relating to all six degrees of freedom in an orthogonal three axis system is shown having a mechanical oscillator constructed from two counterbalanced platforms which form a single unit for rotation about a single torsional axis. A plurality of accelerometers are mounted upon at least one of the counterbalanced platforms. Each accelerometer has an input axis, an output axis, and a pendulous axis. Each accelerometer is mounted such that the input axis is at a predetermined angle to the single axis of rotation and the counterbalanced platform. The accelerometers are used to measure angular rate along the pendulous axis and linear acceleration along the input axis. These signals may then be electronically separated to produce acceleration and angular rate outputs for a three axis orthogonal system.

This invention was made with government support under ContractDAAA21-85-C-0290 awarded by the Department of the Army. The Governmenthas certain rights in this invention.

This is a continuation of copending application Ser. No. 07/315,447,filed Feb. 24, 1989, abandoned, which is a divisional of copendingapplication Ser. No. 07/251,918, filed on Sept. 30, 1988, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an inertial measurement unit (IMU) and,more particularly, to a single IMU which measures angular rate andlinear acceleration for use in a six degrees of freedom, three axisorthogonal system through the use of a single counterbalanced mechanicaloscillator.

2. Description of the Prior Art

Existing strap down inertial measurement units typically consist of acluster of separate gyros and accelerometers of complexelectro-mechanical and/or electro-optical design. These IMUs wereoriginally designed to meet the high performance requirements of anaircraft navigation system with operation times measured in hours.Application of these gyros and accelerometers to flight control IMU'sfor short duration flight, such as required for expendable weapons whichoperate in times measured in minutes, has resulted in systems withexcessive size, weight, power, and cost, and with inadequateenvironmental capabilities. Remember that the existing strap down IMUstypically require a separate gyro and accelerometer for each axis withina three axis orthogonal system.

Some systems have been proposed which reduce the number of gyros andaccelerometers by using one specialized sensor for sensing two axes.Typically, the specialized sensor is placed upon a spinning shaft. Asthe multiple sensor is capable of sensing two of the three axes withinan orthogonal system, there still remains the requirement for two setsof the sensors as well as the need for motors and bearings toaccommodate the spinning axes. This also results in a redundant axiswhich adds size, weight, and cost.

Another approach to an inertial measurement unit for measuring thespecific force and angular velocity of a moving body utilizes anorthogonal triad of rotating accelerometers. This system reduces thenumber of components required to provide an IMU, but there still remainsthe requirement for spinning three accelerometers around the threeorthogonal axes with the accompanying requirement for bearings and spinmotors and resolvers. It has also been suggested that the spinningaccelerometers may be vibrated in an oscillating manner instead ofspinning around the orthogonal axes.

A breakthrough improvement over the arrangements just described may befound in a copending patent application, Ser. No. 045,045, filed May 1,1987, by Robert E. Stewart, entitled "A Miniature Inertial MeasurementUnit", now U.S. Letters Pat. No. 4,841,773 which is assigned to the sameassignee as the present invention. This inertial measurement unitutilizes a single rotor design with six accelerometers mounted at 60°apart on the rim of a paddle-wheel like body. An AC voltage is used todrive piezoelectric strips mounted on the webs of the paddle-wheel likebody resulting in a simple harmonic oscillation of the body. The designproduces a three axis orthogonal measurement system from but onerotating body. This design works well for some applications but has somedifficulty with angular rate performance in the presence of randomvibration.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to produce an improvedIMU which uses a single axis of rotary motion to provide measurements ina three axis orthogonal system.

It is another object of the present invention to improve the angularrate performance of the IMU.

It is still another object of the invention to provide an improved IMUthat uses the Coriolis effect to permit a single inertial sensor, i.e.an accelerometer, to produce both linear acceleration and angular rateoutputs thus eliminating the need for expensive gyroscopes.

It is yet another object of the present invention to provide an IMU witha single counterbalanced mechanical oscillator which minimizes thesensitivity of the mechanism to outside vibrations.

A further object of the present invention is to provide a pair ofaccelerometers which may be utilized to cancel random vibrationalsignals in such a way that the input axis of the pair are collinear ornearly so.

Still a further object of the present invention is to provide twoaccelerometers on the same side of a single counterbalanced mechanicaloscillator to permit the centrifugal forces to be cancelled in anangular rate output channel.

A final object is to provide a structure with three mounting points forthe mechanical oscillator as far from the center of oscillation aspossible for producing a stiff mechanism, stiff against any motion thatis outside of the plane of rotation.

In accomplishing these and other objects there is provided a singlecounterbalanced mechanical oscillator constructed from twocounterbalanced platforms mounted for rotation about a single axis. Aplurality of accelerometers are mounted upon at least one of thecounterbalanced platforms. Each accelerometer has an input axis, anoutput axis, and a pendulous axis. The input axis is arranged at apredetermined angle to the single axis of rotation and to thecounterbalanced platform. A suitable driving device, such apiezoelectric strips, is provide for rotating the two counter-balancedplatforms 180° out of phase.

DESCRIPTION OF THE DRAWINGS

The benefits and advantages of the present invention will be betterunderstood after reference to the following specification and drawings,wherein:

FIG. 1 is a perspective view of an accelerometer used within the presentinvention;

FIG. 2 is a perspective diagram schematically showing the singlecounterbalanced mechanical oscillator of the present invention with itsassociated electronics;

FIG. 3 is a top view showing one embodiment of the singlecounterbalanced mechanical oscillator of the present invention and;

FIG. 4 is a cross-sectional view taken along line 4--4 of FIG. 3

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. shows an accelerometer 10 usefulwithin the inertial measurement unit of the present invention.

It has been observed in the above-referenced Robert E. Stewartapplication, Ser. No. 045,045 and elsewhere that complete six degrees offreedom inertial measurement systems, providing measurements of bothlinear acceleration and angular rate, can be constructed usingaccelerometers as the only inertial sensor. Angular rate can be measuredby accelerometers by sensing Coriolis acceleration. Coriolisacceleration A_(c) is the linear acceleration resultant from the vectorcross product of velocity V with angular rate Ω, i.e., A_(c) =2Ω×V.Physically, this means that if a velocity excitation is applied to theoutput axis OA of a pendulous accelerometer, angular rate about theaccelerometer's pendulous axis PA will be observed as Coriolisacceleration on the instrument's input axis IA as shown in FIG. 1.

Given that Coriolis acceleration makes angular rate observable withlinear accelerometers, the problem of how to separate the linear andCoriolis induced components of measured acceleration must be addressedfor a viable inertial system to be realized.

One approach for separation of linear and Coriolis induced accelerationsis to employ a sinusoidal velocity excitation. As indicated in FIG. 1, asinusoidal velocity excitation V of the form:

    V=V.sub.o sin (ωt)

and an angular rate Ω, along accelerometer's pendulous axis PA, willresult in a sinusoidal Coriolis acceleration A_(c) of the form:

    A.sub.c =2Ω×V=2Ω×V.sub.o Sin ωt

Therefore, as long as there is no input axis linear acceleration at thevelocity excitation frequency, demodulation of the accelerometer'soutput yields a measurement of angular rate about the instrumentpendulous axis PA. Similarly, the component of linear acceleration iseasily obtained by filtering the acceleration's output at the velocityexcitation frequency.

Since a multisensor IMU is required to operate in a vibrationenvironment inclusive of the velocity excitation frequency, a commonmode rejection mechanization is employed. FIG. 2 shows a single channelmechanization using a matched pair of accelerometers 12 and 14 which aremechanically vibrated about a single axis 16 along their output axis OA(shown at V₁ and V₂ in FIG. 2) and moved 180° out of phase with respectto each other on countervibrating platforms 18 and 20.

In the presence of an angular rate Ω along the pendulous axis PA and anacceleration component A along the input axis IA, the total accelerationmeasured by each accelerometer is:

    A.sub.1 (t)=A(t)+2Ω(t)V.sub.o Sin ωt           (1)

    A.sub.2 (t)=A(t)-2Ω(t)V.sub.o Sin ωt           (2)

Subtracting Equations 1 and 2 gives: ##EQU1## Adding Equations 1 and 2gives: ##EQU2## Equations 3 and 4 are the general equations used tocompute linear acceleration A and angular rate Ω. The angular rate isobtained by demodulating Ω(t) sin ωt.

The electronics for determining the linear acceleration and angular rateare shown schematically in FIG. 2 including a first summing circuit 22which adds the acceleration measurements received from accelerometers 12and 14. The output from the summing circuit 22 is applied to a scalingcircuit 24 which, because of the addition of the two accelerationmeasurements, yields a value that is twice the normal magnitude. Thus,the scaling circuit divides the information by two and passes thatinformation to a low pass filter 26 whose output represents linearacceleration A.

Similarly, the outputs from accelerations 12 and 14 are subtracted at asecond summing circuit 28. It will be noted that because the Coriolisvectors which are being subtracted here are in opposite directions.Thus, the difference between the two vectors is twice the Coriolissignal 2Ω×V_(o). A scaling circuit 30 therefore divides the signal fromsumming circuit 28 by 4V_(o) before that signal is applied to andemodulator 32 whose output is the angular rate Ω. An oscillator 34drives the two platforms 18 and 20 180° out of phase with respect toeach other and also applies its signals to the demodulator 32.

Several accelerometer designs may be used in the present invention. Onedesign that may be utilized is disclosed in U.S. Pat. No. 4,679,434,which issued July 14, 1987, by R. E. Stewart. For a more completedescription of the mathematically equations utilized by a single IMU tomeasure a three axis orthogonal system, reference is made to thecopending patent application Ser. No. 045,045, filed May 1, 1987, by R.E. Stewart. Finally, a more complete description of the electronicswhich may be utilized to measure angular rate using Coriolisacceleration and to measure linear acceleration and then to separate thesignals into two usable sets may be found in the copending patentapplication Ser. No. 045,045, by R. E. Stewart.

Referring now to FIGS. 3 and 4, one embodiment of a single IMU formeasuring a three axis orthogonal system is shown at 36. The IMUincludes a cover 38 which is generally shaped as a cylinder closed atone end. The open end of the cover 38 is closed by an electronicshousing 40. The electronics housing 40 includes a plurality of chambers(three of which are shown in the preferred embodiment of FIG. 4) formounting circuitry used in conjunction with the IMU 36. Mounted to theinner surface of the electrical housing 40 is Y-shaped mounting member42 which may be attached to the electrical housing 40 by suitablefastening means, such as screws 44. As seen in FIG. 4, the Y-shapedmounting member 42 consists of two members, one stacked on top of theother, to form the mounting surfaces for the moving portions of the twocounterbalanced platforms 18 and 20 shown in FIG. 1. It will be seen inFIG. 4 that the cross section of the Y-shaped mounting member 42 isprovided with a small standoff 46 which separates the two Y-shapedmembers 42. Extending from the intersection of the legs of the Y-shapedmembers 42 are three webs 48 which are connected to the inner diameterof a toroidally shaped rotor 50. The rotor 50 is relieved at threeequally spaced pads 52 which are equally spaced at 120° about theperiphery of the rotor. Each pad 52 is canted at an angle to the singletorsional axis 16 to receive an accelerometer 10.

It will now be seen that FIG. 3 is a view taken from FIG. 4 with theleft most platform 20 removed to expose platform 18. The rotor 50 ofplatform 18 includes three pads 52 which are canted inwardly so that theouter edge of pad 52 is higher than the inner edge thereof in FIG. 3.

In FIG. 4, it will be seen that the platform 20 includes a rotor 50whose pads 52 have a cant that is lower at its inner edge and slopesupwardly toward the outer edge thereof. In FIG. 4, the reader will notethat the angles on pads 52 are parallel to each other to mount theaccelerometers in parallel to one another. This mounting arrangementpermits the accelerometers on platforms 18 and 20 to form matched pairssuch as the two shown in FIG. 4. The reader will now understand that thearrangement of the accelerometers 10 causes the input axis IA, outputaxis OA, and pendulous axis PA to be mounted in parallel to each other.Suitable wiring terminals 54 are arranged in the electronic housing 40for connecting the electronics shown in FIG. 2 to the accelerometers 10.

These wire terminals 54 are also used to connect a plurality ofpiezoelectric drivers 56 formed as thin strips which are mounted onopposite sides of each web 48. The piezoelectric strips 56 are driven byan electric signal such that the strip on one side of web 48 is causedto expand while the strip on the opposite side is caused to contract.This produces a dither-like rotation of the platforms 18 and 20 aboutthe single torsional axis 16. As each platform 18 and 20 is driven 180°out of phase from the other, it will be understood that the rotationalforce exerted by platform 18 is counterbalanced by the rotational forceexerted by platform 20. This arrangement significantly reduces thevibration coupled to the housing 40 that could be caused by a singlerotational element. The single counterbalanced mechanical oscillator ofthe present invention is thus the torsional equivalent of a tuning forki.e., each platform 18 and 20 is driven at the same natural frequency.

The pendulous axes PA of each set of three accelerometers 10 mountedupon the rotor platform of FIG. 4 are arranged orthogonally to eachother. That is, the pendulous axes PA are arranged so that the axes formthe edges of a cube whose corner, formed by the edges, is trisected bythe torsional axis. In the embodiment shown in FIG. 4 the accelerometerscan be rotated about their input axes IA so that the output axis OA andpendulous axis PA can be interchanged or arranged at any angletherebetween. Only one of the rotor platforms in FIG. 4 requireaccelerometers in an environment that is vibration free, such as space.However, in the preferred embodiment, the accelerometers are mounted onboth rotor platforms so that the mating pairs of accelerometers maycancel external, unwanted vibrations while doubling the angular rate andacceleration signals. The cancellation of unwanted vibration permits thedither frequency to be used to sense information including angular rateΩ and linear acceleration A that permits the tracking of an object uponwhich the IMU is mounted. Electronics capable of driving the IMUs ofFIG. 4 are shown in the copending patent application Ser. No. 045,045,by R. E. Stewart now U.S. Letters Pat. No. 4,841,773. The only majordifference is that the dither driver shown therein generates a sine anda cosine signal for driving the two platforms 18 and 20 or 70 and 72180° out of phase.

It will be understood that other variations of the single axis IMU formeasuring a three axis orthogonal system are possible within theteachings of the present invention which should be limited only by theappended claims.

We claim:
 1. A three axis inertial measurement unit, comprising:twocounterbalanced mechanisms each mounted for mechanical oscillation at anatural frequency about a single axis wherein one mechanismcounterbalances the other; said two counterbalanced mechanisms mountedby mounting means for imparting rotational motion thereto only in arespective plane of rotation perpendicular to said single axis; aplurality of accelerometers mounted on at least one of saidcounterbalanced mechanisms; each accelerometer having at least an inputaxis; said plurality of accelerometers mounted upon said at least onecounterbalanced mechanism such that said input axis of eachacceleromoter is at a predetermined angle to said single axis and tosaid single plane of rotation of said counterbalanced mechanisms.
 2. Theinertial measurement unit of claim 1, additionally comprising:saidrotational motion of said counterbalanced mechanisms is a ditheredrotational motion; said single axis is a torsional axis; and drive meansfor driving each of said counterbalanced mechanisms at a ditheredoscillation that is 180° out of phase.
 3. The inertial measurement unitof claim 2, additionally comprising:said plurality of accelerometersmounted upon said at least one counterbalanced mechanism include threeaccelerometers mounted upon said at least one mechanism for providinginertial measurements in a vibration-free environment.
 4. The inertialmeasurement unit of claim 2, additionally comprising:said plurality ofaccelerometers mounted upon at least one counterbalanced mechanisminclude six accelerometers with three accelerometers mounted upon eachcounterbalanced mechanism for providing inertial measurements in avibrating environment.
 5. The inertial measurement unit of claim 4,additionally comprising:said three accelerometers mounted upon one ofsaid two counterbalanced mechanisms form matched pairs with said threeaccelerometers mounted upon the second of said counterbalancedmechanisms, such that said input axis of each matched accelerometer pairis parallel to one another and at a predetermined angle to said singletorsional axis.
 6. The inertial measurement unit of claim 2,additionally comprising:said accelerometers are silicon accelerometers;and said drive means are piezoelectric strips mounted upon said mountingmeans for said counterbalanced mechanisms.
 7. A three axis inertialmeasurement unit, comprising:two counterbalanced platforms each mountedfor mechanical oscillation at a natural frequency about a singletorsional axis wherein the first platform counterbalances the second; aplurality of accelerometers mounted on at least one of said platforms;each accelerometer having at least an input axis, said plurality ofaccelerometers mounted upon said at least one platform such that saidinput axis of each accelerometer is at a predetermined angle to saidsingle torsional axis and said platform; said two counterbalancedplatforms each having:a Y-shaped mounting member; a rotor membersurrounding said Y-shaped mounting member; and web members joining saidY-shaped mounting member to said rotor member; said accelerometersmounted upon said at least one of said platforms mounted upon said rotormember thereof at said predetermined angle.
 8. The inertial measurementunit of claim 7, additionally comprising:said plurality ofaccelerometers include six accelerometers having three each mounted 120°apart upon each rotor member and arranged in matched pairs with saidinput, output, and pendulous axes of each pair mounted in parallel toeach other and at predetermined angles to said single torsional axis. 9.The inertial measurement unit of claim 7, additionallycomprising:piezoelectric drivers mounted upon said web means to drivesaid two counterbalanced platforms 180° out of phase from each otherduring said dithered rotation thereof.
 10. The inertial measurement unitof claim 1, additionally comprising:said accelerometers each having aninput axis, an output axis, and a pendulous axis.
 11. The inertialmeasurement unit of claim 7, additionally comprising:said accelerometerseach having an input axis, an output axis, and a pendulous axis.